For many years, mechanical watch movements have undergone multiple improvements, particularly in order to adjust or regulate the oscillation frequency of the sprung balance used as the resonator for the local oscillator. The conventional mechanical watch movement and, in particular, its Swiss lever escapement, is characterized by its robustness to shocks experienced by the watch. This means that the state of the watch is, in general, unaffected by a one-time shock. However, the efficiency of such an escapement is not very good, for example around 30%. Further, the Swiss lever escapement does not permit the use of resonators with a high frequency or low amplitude.
WO Patent Application No. 2006/045824 A2 discloses a regulating member for returning the oscillating sprung balance to a position of equilibrium. An escapement is also provided for maintaining the oscillation of the balance wheel about its position of equilibrium. To achieve this, the balance wheel is connected to at least one movable permanent magnet, whereas the regulating member has a fixed permanent magnet so as to generate a magnetic field returning the balance to its position of equilibrium. There is no description regarding a feedback system capable of adjusting the oscillation frequency of the sprung balance, which constitutes a drawback.
In order to maintain a local oscillator resonator at a high frequency, the principle of the Swiss lever escapement has to be adjusted. To achieve this, an increase in the frequency of the regulating member requires more energy to maintain the oscillator. To reduce energy, it is possible to reduce the oscillator mass or inertia, to reduce the oscillation amplitude, to increase the quality factor of the oscillator, or to improve the energy transmission efficiency between the drive member and the regulating member. Thus, with a conventional Swiss lever escapement, too much energy is consumed by accelerating and stopping multiple times per second. Even if the Swiss lever and its wheel are made as light as possible, this does not make it easy to achieve a high frequency oscillator.
In a mechanical movement proposed by De Bethune, there is proposed a magnetic escapement with a continuous sinusoidal transmission of energy. A mechanical drive member transmits a torque force to a reduction gear train. At the end of said gear train, a magnetic rotor transmits energy to the resonator of the local oscillator, on which permanent magnets are fixed. The gear train speed is synchronized with the natural resonator frequency. The resonator, as the regulating member, controls the measurement of time. The speed of motion of the time indicator hands is controlled by a precise and regular division of time.
Such a resonator can replace the conventional sprung balance to better satisfy the requirements and constraints of high frequency oscillation in order to improve precision. There are no longer any specific points of attachment. The resonator is stiffer and permits the use of a first natural vibration mode. The quality factor is also higher than a conventional oscillator even at low amplitude.
However, according to the embodiment described above for the De Bethune movement, there is no shock resistance. In such conditions, the hands are likely to advance rapidly after any shocks. Further, there is no description of a feedback system for simply and precisely adjusting the oscillation frequency of a sprung balance, as in the present invention, which constitutes a drawback.
The invention seeks a means for using a local oscillator resonator, which has a high quality factor, a high frequency and/or low amplitude. This is achieved without relinquishing the robustness to shocks of a Swiss lever escapement.